maybe there could be a slime alien that leaves a trail and if any of your crew steps on it that slip and damage themselves.

And some type of giant bug alien that when you battle you can detain one and try and convert it into a pet.

Also there is a special mission that a alien parasite infects one of your crew and it transforms them into a mutant robot so you have to take it to its home planet and try and find it a new host but end up being involved in some national war were you can either help the parasite or flee the planet and doom it to being corrupted .

Plus a race of aliens that are just shadows but they can remain invisible until they attack

Finally a race of living rock that cannot shot but excel at close ranged combat